The use of bicarbonate salts (baking soda) as a dentifrice or the incorporation of such salts into dentifrice compositions is well known in the art of oral care. A renewed interest in incorporating bicarbonate salts into toothpaste has emerged in light of the success of the present assignee's Dental Care® and PeroxiCare® products. The addition of bicarbonate salts into dentifrices is beneficial for several reasons such as for providing good plaque removing capabilities, as well as for improving the whitening properties of dentifrices. Importantly, bicarbonate salts provide a clean fresh feeling in the oral cavity after brushing and rinsing with water.
In formulating toothpastes containing high levels of sodium bicarbonate, one of the difficulties can be the incorporate of certain anions, which are added to bestow on the formulation certain benefits. The concentration of sodium ions present in the liquid phase of the toothpaste can lead to salting out of these anions as crystal hydrates of their sodium salts. Often these hydrated sodium salts undesirably precipitate as large crystals.
Thus, for example, in formulating a sodium bicarbonate-containing tartar control toothpaste it is desirable to add a pyrophosphate salt. Tartar, known also as calculus, is a hard mineralized deposit which forms around teeth. This formation arises from deposition of crystals of calcium phosphate in the pellicle and the extra-cellular matrix of dental plaque. Various forms of calcium phosphate have been identified but the most difficult to remove and thermodynamically most stable form is called hydroxyapatite (HAP). Amorphous forms of calcium phosphate are believed to be the precursors of HAP. Regular brushing can usually remove the amorphous forms but is not fully effective to dislodge the final stable calculus form. Therefore it is desirable to prevent amorphous forms of calcium phosphate from transforming into HAP. The art has recognized that agents which interfere with the formation of HAP crystallization will be effective anti-tartar agents.
Soluble inorganic pyrophosphate salts have over the last decade set the commercial standard as tartar control agents. This technology has been reported by Parran, Jr. et al. in a series of patents including U.S. Pat. No. 4,590,066, U.S. Pat. No. 4,515,772 and U.S. Pat. No. 4,684,518.
However, in the presence of high concentrations of sodium ions provided by the sodium bicarbonate, disodium dihydrogen pyrophosphate hexahydrate or tetrasodium pyrophosphate decahydrate can precipitate as large crystals.
Similarly, in formulating a bicarbonate-based toothpaste containing a remineralization promoting system, orthophosphate is required. The primary component of the enamel and dentin in teeth is calcium phosphate in the form of calcium hydroxyapatite. This material is highly insoluble at normal oral pHs. However, carious lesions (demineralization) form in teeth when the teeth are subjected to acids produced from the glycolysis of sugars by the action of various oral bacteria. This is because calcium phosphate salts are more soluble in acidic media.
Saliva is supersaturated with respect to calcium and phosphate ions. Saliva therefore helps protect teeth against demineralization and can slowly remineralize teeth which have become demineralized by acids. It is well known that the presence of fluoride ions can enhance the natural remineralization process and this is one of the accepted mechanisms by which fluoride toothpastes and rinses protect against caries. The efficacy of fluoride-containing toothpastes and rinses to remineralize teeth is limited by the modest levels of calcium and phosphate in saliva. It is evident from the prior art that it is highly desirable to increase the available concentration of calcium and phosphate ions in the oral cavity to speed up the remineralization process. However, because of calcium phosphate's low solubility at the pH of saliva, the addition of higher levels of dissolved calcium and phosphate ions is not easily accomplished.
Oral products designed to remineralize subsurface lesions in teeth and mineralize exposed dentinal tubules, i.e., remineralizing/mineralizing products, are disclosed, for example, in U.S. Pat. No. 5,603,922; 5,605,675; 6,159,448; 5,833,957; and 5,858,333, all to Winston and Usen. It is believed that the first remineralizing dentifrice made available to the public was based on the Winston and Usen patents. These patents describe a two-part product comprising a first part comprising a calcium salt and a second part comprising phosphate and fluoride salts which are kept separate in a tube by a physical divider but wherein both parts are dispensed simultaneously from the tube on the tooth brush applicator. In a bicarbonate-based remineralizing toothpaste, the calcium component would be separated from the fluoride, phosphate and bicarbonate components as two phases. The calcium phase would contain a calcium salt and other conventional toothpaste ingredients while the other phase would contain phosphate, bicarbonate and fluoride sources in a base of water, humectants and other toothpaste ingredients. Unfortunately, problems arise because the addition of phosphate salts to the phase containing sodium bicarbonate and fluoride can result in the formation of large crystals of hydrated mono, di, or trisodium orthophosphate sometimes also containing fluoride ions.